In mechanical seals used for pumps in general industrial applications (general purpose pumps), a sealing surface is formed between a stationary ring provided on a seal cover that is fixed to a housing of a pump body (stuffing box) and a rotating ring provided on a collar that is fixed to a rotating shaft. The seal cover is typically secured to an end face of the housing by bolts with a gasket in between. A housing end face generally has two or four fastening points (bolt holes or stud bolts) disposed at regular intervals in a circumferential direction of a shaft hole but fastening positions of these fastening points (radial positions in relation to the center of the shaft hole) may differ from one pump to another even if the diameter of rotating shafts are same. Thus, for conventional mechanical seals, seal covers having four U-shaped grooves disposed at regular intervals in a circumferential direction have been used in order to give them a versatility to be used in various pumps with different fastening positions (See Patent Document 1).
A seal cover of a mechanical seal according to prior art will be described below with reference to FIG. 4. FIG. 4 is a front view of a conventional mechanical seal. The seal cover 90 included in the mechanical seal 900 has four U-shaped grooves 91 that are disposed at regular intervals in a circumferential direction around the shaft hole. The U-shaped grooves 91 are extended as radially inward as possible so as to widen a range where bolt shanks can be passed through. Because the seal cover 90 is configured in such a way, it can be secured to either of pumps having two fastening points or four fastening points on their housing end faces, and a range of securable fastening positions becomes wider. Hence, its versatility is high.
However, when the seal cover 90 is secured at two U-shaped grooves 91 located at symmetrical positions about the center of the shaft hole, a deformation may be caused because of the fastening forces of the bolts. This causes a gasket to be compressed insufficiently and eventually lowers a sealing performance. Such deformation is particularly significant in the vicinity of the other two U-shaped grooves 91 where no bolts are fastened. The deformation is also particularly significant when the seal cover with a large shaft hole is secured. A thickness (a thickness in an axial direction) of the seal cover 90 may be increased in order to give a sufficient strength but other problems such as a weight increase or a spatial restriction at installation sites may arise.